1. Field of the Invention
The invention relates to railroads generally, and more particularly to a method and system for detecting when an end of train passes a point such as a mile marker, switch, siding or other location of interest.
2. Discussion of the Background
It is often important to be able to determine that a railroad has passed a particular point in a railroad. For example, in a train control method known as Track Warrant Control (TWC), a railroad is divided into sections referred to as blocks and a dispatcher gives each train warrants, or authorities, to occupy and/or move in one or more blocks. The blocks are usually (but not necessarily) fixed, with block boundaries usually (but not necessarily) being identified with physical locations on the railroad such as mileposts, sidings, and switches. In this system, a train in a first block (or group of blocks) receives a warrant to occupy a second adjacent block (or group of blocks) from the dispatcher and informs the dispatcher when it has cleared the first block and has entered the following block. After the train notifies the dispatcher that the first block has been cleared, the dispatcher may issue an unrestricted (rather than a “joint” or “permissive” warrant) warrant to occupy the first block to a second train. If such a warrant to occupy the first block is issued to the second train before the end of the first train has cleared that block, a collision between the two trains may result. Therefore, determining that the end of the train has left a block is critical in a track warrant control system.
As another example, it may be necessary to wait until one train has passed a switch so that the switch position can be set in a different direction for a following train. There are yet other examples in which it is necessary to determine that an end of train has passed a point such as the end of a block.
Determining that an end of a train has passed a point is not a trivial process. Modern trains can be hundreds of yards long, and an engineer in the lead locomotive often cannot see the end of the train. Operating trains at night or during bad weather may also make visually determining that the end of a train has passed a point difficult or impossible. Thus, visual methods are not sufficient.
A second method used to determine that the end of a train has passed a point is to determine how far the head of the train has traveled past the point using a wheel tachometer/revolution counter or a positioning system (e.g., a GPS system). With this method, once the head of the train has traveled a distance equal to the length of the train past the point, it is assumed that the end of the train has passed the point. However, with this method, it is important to take into account the possibility that one or more end cars of a train may become uncoupled from the remainder of the train.
One way in which uncoupled cars can be detected is through the use of end-of-train, or EOT, devices equipped with motion detectors. These devices, which communicate via radio with the head of the train (HOT), provide an indication as to whether or not the end of the train is in motion. However, with these devices the motion sensors sometimes break or give false readings and, under certain circumstances, may mislead a conductor or engineer even when working properly. One potentially disastrous incident known to the inventors in which even a properly functioning motion detector can give a misleading indication involves a distributed power train. A distributed power train is a train comprising one or more locomotives placed at the front of the train, followed by one or more cars, followed by one or more additional locomotives and cars. In such a train, the throttles in the second group of locomotives are operated by remote control to be in the same position as the throttles in the first group.
In the above-referenced incident, a distributed power train was temporarily stopped at a crossing. While stopped, a vandal disconnected the second group of locomotives from the preceding car and closed off the valves in the air brake line (had these valves not been closed off, a failsafe mechanism would have activated the brakes to prevent the train from moving). In this particular distributed power train, the second group of cars connected to the second group of locomotives was heavier than the first group of cars connected to the first group of locomotives. Because the second group of cars was heavier than the first, there was a difference in speed between the two portions of the train when the train began moving after being uncoupled by the vandal, and the first portion of the train began to separate from the second portion. The EOT motion sensor transmitted the correct status that the EOT (last car) was moving, but did not (indeed, could not) indicate the train was separated. In this incident, the separation grew to over a mile before the engineer noticed that there was a problem.
If the engineer on this train had relied on the distance traveled by the head of the train to report to the dispatcher that the end of the train had cleared the previous block, then an extremely dangerous situation would have resulted in that the end of the separated train would still have been in the previous block where an oncoming train might have collided with it. Thus, any method used to determine that the end of the train has passed a point should take into account the possibility that the end of the train may have become separated from the head of the train.
One method for detecting that a train has passed a point is discussed in U.S. Pat. No. 6,081,769. In this method, discussed at col. 4, lines 49-67, a second GPS receiver is placed on the end of the train and the position reported by that receiver is used to determine that the end of the train has passed the point of interest. This patent also discloses that the difference in position reported by the first and second GPS receivers can be used to determine the length of the train.